BAROTROPIC PHASE-TRANSITIONS OF DIOLEOYLPHOSPHATIDYLCHOLINE AND STEAROYL-OLEOYLPHOSPHATIDYLCHOLINE BILAYER-MEMBRANES

In order to understand the effect of cis unsaturation on the thermotro pic and barotropic phase behavior of phospholipid bilayer membranes, t he phase transitions of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC ), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), 1,2-distear oyl-sn-glycero-3-phosphocholine (DSPC) bilayer membranes were observed by high-pressure optical method. With respect to DOPC bilayer membran e, the so-called main transition between the liquid crystalline (L-alp ha) and the lamellar gel (L-beta) phases was observed in water at abov e 0 degrees C under high pressure, in addition to the transition betwe en the L-alpha and the lamellar crystalline (L-beta) phases in 50% aqu eous ethylene glycol. The pressure of main transition increased linear ly with an increase in temperature. Extrapolation of temperature (T)-p ressure (P) phase boundary to ambient pressure suggests the temperatur e of the main transition to be -40.3 degrees C, which has never been f ound by the DSC method. On the other hand, the temperature of L-C/L-al pha phase transition in 50% aqueous ethylene glycol was found to be -1 2.0 degrees C at ambient pressure. The main transition temperatures fo r DSPC, SOPC and DOPC are 55.6, 6.7 and -40.3 degrees C, respectively, at ambient pressure. The substitution of cis unsaturated chain for sa turated chains of DSPC brings about the depression of the main transit ion temperature by about 48 (+/- 1)degrees C for each chain. The volum e changes (Delta V) associated with the transitions were calculated fr om the transition enthalpy (Delta H) and the slope of T-P diagram (dT/ dP) by means of the Clapeyron-Clausius equation. The value of Delta V for the main transition of SOPC bilayer membranes was reduced to half the volume change for DSPC bilayers, which means the introduction of t he cis double bond in the acyl chain of lipids brings about the reduct ion of Delta V because of the disordered packing of unsaturated chains in the gel phase of lipid bilayer membranes. 0005-2736/98/$ - see fro nt matter (C) 1998 Elsevier Science B.V. All rights reserved.